Magnet roller developing device and image forming apparatus for reducing obstructions in developer circulation path

ABSTRACT

A device including a developing roller having a magnet roller for transporting developer, and developing an electrostatic latent image, a regulating member regulating transportation amount of the developer transported to the electrostatic latent image with the development roller, and a reflux plate refluxing excess developer regulated by the regulating member away from the regulating member is constructed to comprise a rubbing member arranged to face the developing roller, at a second gap larger than a first gap defined by the developing roller and the regulating member, and rubbing developer transported to the first gap. The rubbing member carries out preliminary electrification on developer and then the regulating member carries out main electrification on the preliminarily electrified developer.

This application is the U.S. national phase of international applicationPCT/JP2004/012992 filed 7 Sep. 2004 which designated the U.S. and claimspriority to JP 2003-317562 filed 9 Sep. 2003, and JP 2004-038375 filed16 Feb. 2004, the entire contents of each of which are herebyincorporated by reference.

TECHNICAL FIELD

Example embodiments of the present invention relate to a developingdevice used for an image forming apparatus employing anelectrophotographic system, such as a copier, printer, and facsimilemachine, and the image forming apparatus.

BACKGROUND ART

FIG. 13 is a cross-sectional view showing a structure of a conventionaldeveloping device.

A developing device used for an image forming apparatus employing anelectrophotographic system such as a copier includes a developing roller101 for having a magnet roller 100 which transports two-elementdeveloper and developing electrostatic latent images held by aphotosensitive drum, a regulating member 102 for regulating an amount ofthe developer transported to the electrostatic latent image with thedeveloping roller 101, a reflux plate 103 for refluxing excess developerwhich is regulated by the regulating member 102 away from the regulatingmember 102, a hopper 104 in which the developer is contained, and aagitating roller 105 for agitating the developer in the hopper 104 (forexample, see Japanese Patent Application Laid-Open No. 1-237577 (1989)).

The developing roller 101 includes the nonrotational magnet roller 100in which magnetic poles N and magnetic poles S are alternativelyarranged in the circumferential direction thereof and a nonmagneticsleeve 106 which is rotatably fitted to the magnet roller 100.

The regulating member 102 is arranged near a developing range facing thedeveloping roller 101, and the reflux plate 103 is provided at theopposite side of the regulating member 102 across the developing range.

According to the developing device having such structure, two-componentcarrier (magnetic power) and toner contained in the hopper 104 areagitated by the agitating roller 105 and toner is attached tocircumference of the carrier. Further, as the sleeve 106 of thedeveloping roller 101 rotates in opposite direction of the rotation ofthe photosensitive drum, developer between the developing roller 101 andthe reflux plate 103 is transported toward a gap and the amount of thedeveloper transported to the electrostatic latent image is regulated bythe regulating member 102. The developer passed through the gap istransported to the electrostatic latent image. The excess developerregulated by the regulating member 102 accumulates in an interspacebetween the regulating member 102 and the reflux plate 103. As theaccumulation amount increases, the developer flows toward upper surfaceof the reflux plate 103, is guided by the reflux plate 103, and refluxestoward the upper side of the agitating roller 105.

As developing devices in which two-component developer is employed, adeveloping device including a refluxing unit for refluxing excessdeveloper which is regulated by a regulating member toward the upperside of an agitating roller and a reflux plate having a folded portionin one end of the refluxing unit is also well known (for example, seeJapanese Patent Application Laid-Open No. 3-89273 (1991)).

However, there have been problems in the developing device disclosed inH No. 1-237577 (1989) having a structure in which the reflux plate 103for refluxing excess developer away from the regulating member 102 isprovided. The developer is transported to the gap without preliminaryelectrification in positive manner and the transportation amount of thedeveloper is regulated by the regulating member 102. Accordingly,carrier of the developer transported to the gap is in state of chaincomposition with relatively strong force by the magnetic attractionforce of the magnet roller 100. The part of the developer in chaincomposition state becomes excess developer by the regulation of theregulating member 102. As the excess developer increases, a ball of thedeveloper in chain composition state may grow. The grown ball ofdeveloper drops into the downstream side of the reflux plate 103 becauseof the own weight, and thus, a developer circulation path toward thedeveloper supply unit in the hopper 104 is not formed. Further, thenumber of turning on and off of the developer magnetic permeabilitysensor provided at the bottom of the hopper 104 increases so that thechanges in electrification amount of the developer increases.

DISCLOSURE

Example embodiments have been made in view of the above problems and hasan object of providing a developing device and an image formingapparatus capable of refluxing excess developer generated by aregulating member smoothly by providing a rubbing member for facing adeveloping roller at a second gap larger than a first gap between adeveloping roller and a regulating member and rubbing (rubbed friction)the developer with a regulation of layer thickness of developertransported to the first gap.

Another object of example embodiments is to provide a developing deviceand an image forming apparatus capable of maintaining a stable conditionin which excess developer does not easily bind to each other byproviding one of magnetic poles included in a magnet roller so that, ata position facing the second gap, a polar central axis thereof is towardthe regulating member with respect to the position displaced 1.5 degreesaway from the regulating member from a minimum position of the secondgap.

Another object of example embodiments is to provide a developing deviceand an image forming apparatus capable of maintaining a stable conditionin which excess developer does not easily bind to each other byproviding one of magnetic poles included in a magnet roller so that, ata position facing the second gap, a polar central axis thereofcorresponds to a mininmm position of the second gap.

Another object of example embodiments is to provide a developing deviceand an image forming apparatus capable of maintaining a stable conditionin which excess developer does not easily bind to each other byproviding one of magnetic poles included in a magnet roller so that, ata position facing the second gap, a polar central axis thereof islocated toward the regulating member with respect to the minimumposition of the second gap.

Another object of example embodiments is to provide a developing deviceand an image forming apparatus for an efficient preliminaryelectrification by defining a second gap G2 as a relation of first gapG1<second gap G2≦0.8×width of magnetic pole Dm.

Another object of example embodiments is to provide a developing deviceand an image forming apparatus capable of circulating developer smoothlyby providing a developer accumulation preventing member for preventingaccumulation of developer transported to the gap near a regulatingmember and/or rubbing member.

Another object of example embodiments is to provide a developing deviceand an image forming apparatus capable of reducing displacement of arubbing member toward a gap expansion direction because of dragresulting from rubbing developer by forming a rubbing member with areflux plate integrally.

Another object of example embodiments is to provide a developing deviceand an image forming apparatus capable of reducing curvature of arubbing member toward a gap expansion direction because of dragresulting from rubbing developer by providing a convex portion, to adeveloper accumulation preventing member provided near a regulatingmember, which contacts with the reflux plate and prevents the bending ofthe rubbing member.

Another object of example embodiments is to provide a developing deviceand an image forming apparatus capable of reducing difference of degreesof deformation of a regulating member and a rubbing member caused bytemperature changes by providing the regulating member and the rubbingmember which have similar coefficient of linear expansion.

Another object of example embodiments is to provide a developing deviceand an image forming apparatus capable of maintaining an accuratepositional relation among a developing roller, regulating member, andrubbing member by supporting the developing roller, regulating member,and rubbing member with a common supporting member.

Another object of example embodiments is to provide a developing deviceand an image forming apparatus capable of increasing circulation amountwith preliminary electrification by defining developer amount M2transported to a second gap to be M2>(M1/G1)G2, when developer amounttransported from a first gap is defined as M1.

Another object of example embodiments is to provide a developing deviceand an image forming apparatus capable of reducing resistance ofdeveloper circulation at a third gap defined by a reflux plate and adeveloper accumulation preventing mdmber provided near a regulatingmember, and preventing reduction of developer circulation amount causedby circulation path resistance, by setting the third gap to be equal toor larger than 4.2 mm.

Another object of example embodiments is to provide a developing deviceand an image forming apparatus capable of reducing resistance ofdeveloper circulation at the third gap and preventing reduction ofdeveloper circulation amount caused by circulation path resistance, byproviding a third gap -larger than a second gap.

Another object of example embodiments is to provide a developing deviceand an image forming apparatus capable of increasing proper tonerconcentration of the developer and improving the image quality, byproviding a hopper containing developer magnetic powder having averageparticle size smaller than or equal to 65μm and toner having averageparticle size smaller than or equal to 7.5μm.

A developing device and an image forming apparatus according to exampleembodiments is a developing device and an image forming apparatusincluding a developing roller having a magnet roller for transportingdeveloper, and developing an electro static latent image, a regulatingmember regulating transportation amount of the developer transported tothe electrostatic latent image with the development roller, and a refluxplate refluxing excess developer regulated by the regulating member awayfrom the regulating member, the device comprising: a rubbing memberarranged to face the developing roller, at a second gap which is largerthan a first gap defined by the developing roller and the regulatingmember, and rubbing developer transported to the first gap.

According to example embodiments, since the second gap is larger thanthe first gap, excess developer is certainly generated by the regulatingmember and circulation path of the excess developer can be formed. Inaddition, before developer is transported to the first gap, the rubbingmember regulates layer thickness of the developer to rub, that is,provide rubbed friction, and preliminary electrification is performed onthe developer. Accordingly, when the developer is two-componentdeveloper, repulsive force caused by the regulating member operates inthe excess developer. The binding force in the developer can be reducedby the repulsive force and the developer is hardly bound to each other.As a result, the excess developer generated by the regulating member issmoothly refluxed toward the reflux plate, the developer is refluxedtoward the start side of the circulation by the reflux plate, and thedeveloper can be circulated smoothly.

A developing device and an image forming apparatus according to exampleembodiments is characterized in that the magnet roller has a pluralityof magnetic poles arranged in circumferential direction thereof and oneof the magnetic poles is provided so that, at a position facing thesecond gap, a polar central axis thereof is positioned toward theregulating member with respect to the position displaced 1.5 degreesfrom a minimum position of the second gap in the direction away from theregulating member.

According to example embodiments, since a polar central axis of themagnetic pole is positioned toward the regulating member from a minimumposition of the second gap, the transportation amount of the developertransported to the second gap itself can be reduced and excess developeramount generated by the regulating member can be reduced. As a result,the excess developer can be maintained in a condition such thatdeveloper is hardly bound to each other.

A developing device and an image fonning apparatus according to exampleembodiments is characterized in that the magnet roller has a pluralityof magnetic poles arranged in circumferential direction thereof and oneof the magnetic poles is provided so that, at a position facing thesecond gap, a polar central axis thereof corresponds to a minimumposition of the second gap.

According example embodiments, since a polar central axis of themagnetic pole corresponds to the minimum position of the second gap, thetransportation amount of the developer transported to the second gap iskept from too much increase and excess developer generated by theregulating member can be maintained in a condition that developer ishardly bound to each other.

A developing device and an image forming apparatus according to exampleembodiments is characterized in that the magnet roller has a pluralityof magnetic poles arranged in circumferential direction thereof and oneof the magnetic poles is provided so that, at a position facing thcsecond gap, a polar central axis thereof corresponds to a minimumposition of the second gap.

According to example embodiments, since a polar central axis of themagnetic pole corresponds to the minimum position of the second gap, thetransportation amount of the developer transported to the second gap iskept from too much increase and excess developer generated by theregulating member can be maintained in a condition that developer ishardly bound to each other.

A developing device and an image forming apparatus according to anexample embodiment is characterized in that the second gap G2 (mm) isdefined asG1<G2≦0.8×Dm

where G1 is the first gap (mm) and Dm is width of magnetic pole (mm).

According to example embodiments, since the second gap is larger thanthe first gap, main electrification can be performed on the developertransported to the second gap by the regulating member after apreliminary electrification is performed by the rubbing member. Further,since it is defined as G2≦0.8×Dm, a state of high magnetic flux densitynear the reflux plate, that is, magnetic attraction force proportionalto the square of the magnetic flux density is added to the carrier ofthe two-component developer. The layer thickness is regulated at the endof lower transportation stream at the rubbing member in a conditionunder relative large frictional force and preliminary electrificationcan be performed efficiently. Further, preferably, it is defined as G2(⅔)×Dm to act as a condition that the magnetic flux density near therubbing member is relatively high, that is, magnetic attraction forcewhich is proportional to the square of the magnetic flux density on thecarrier. The layer thickness is regulated at the end of lowertransportation stream at the rubbing member in a condition under largerfrictional force and preliminary electrification can be performed muchmore efficiently.

A developing device and an image forming apparatus according to the anexample embodiment is characterized by further comprising a developeraccumulation preventing member preventing accumulation of developertransported to the gap near the regulating member and/or the rubbingmember.

According to example embodiment, since the developer accumulationpreventing member is provided near the regulating member and/or therubbing member, the magnetic flux density in the circulation path of theexcess developer generated by the regulating member can be reduced andthe developer is circulated smoothly. In addition, deterioration of thedeveloper by unnecessary stress of developer accumulation can beprevented.

A developing device and an image forming apparatus according to anexample embodiment is characterized in that the rubbing member isintegrally formed with the reflux plate.

According to example embodiment, since the rubbing member and the refluxplate reinforce each other, bending of the rubbing member toward gapexpanding direction caused by a drag of rubbing the developer can bereduced and the amount of the second gap is maintained. Further, bendingof the reflux plate can be reduced, so that circulating ability of thedeveloper is maintained.

A developing device and an image forming apparatus according to anexample embodiment is characterized in that the developer accumulationpreventing member provided near the regulating member has a convexportion which contacts with the reflux plate and prevents thedisplacement of the rubbing member by developer.

According to example embodiment since the convex portion of thedeveloper accumulation preventing member contacts with the reflex plateto regulate bending of the rubbing member caused by developer, bendingof the rubbing memember toward gap expanding direction caused by a dragof rubbing the developer can be reduced and the amount of the second gapis maintained. Further, bending of the reflux plate can be reduced, sothat circulating ability of thc developer is maintained.

A developing device and an image forming apparatus according to exampleembodiment is characterized in that the regulating member and therubbing member have similar coefficient of linear expansion.

According to example embodiment, since difference in deformation of theregulating member and the rubbing member by temperature changes can bereduced, deformation of the first gap and the second gap can be reduced.Accordingly, stable first gap and second gap can be obtained.

A developing device and an image forming apparatus according to anexample embodiment is characterized in that the developing roller, theregulating member, and the rubbing member are supported by a commonsupporting member.

According to example embodiment, since the accurate positional relationamong the developing roller, the regulating member, the rubbing membercan be maintained, the amount of the first gap and second gap can bemaintained. Further, the developing roller, the regulating member, therubbing member can be integrated into a unit so that working propertyfor assembly is improved.

A developing device and an image forming apparatus according to anexample embodiment is characterized in that amount of developertransported to the second gap M2 (g/s/cm) is defined asM2>(M1/G1)G2

where G1 is the first gap (mm), G2 is the second gap (mm), and M1 isamount of developer transported from the first gap (g/s/cm.).

According to an example embodiment, since the developer transported intothe second gap is not regulated in its layer thickness, the layerthickness is not even.

Accordingly, even when the average of transportation amount M2av islower than or equal to the second gap G2·(M1/G1), a part of thedeveloper contacts with the regulating member to be rubbed so thatcirculation amount by preliminary electrification can be increased.

A developing device and an image forming apparatus according to anexample embodiment is characterized in that a third gap which is definedby the reflux plate and the developer accumulation preventing memberprovided near the regulating member is equal to or larger than 4.2 mm.

According to example embodiment, the circulation resistance of thedeveloper at the third gap is reduced so that the reduction of thedeveloper circulation amount caused by circulation path resistance canbe controlled.

A developing device and an image forming apparatus according to anexample embodiment is characterized in that the third gap is larger thanthe second gap.

A developing device and an image forming apparatus according to the anexample embodiment is characterized by further comprising a hoppercontaining developer including magnetic powder in which average particlesize is smaller than or equal to 65 μm and toner in which averageparticle size is smaller than or equal to 7.5 μm.

According to example embodiment, when the particle size of the developeris reduced to improve the image quality, it is particularly effectivesince proper toner concentration of the developer becomes high, magneticattraction force is reduced, and more developer drops just below theregulating member.

As described above, according to an example embodiment, developer isrubbed by the rubbing member in advance to transport to the first gapand preliminary electrification is performed on the developer.Accordingly, when the developer is two-component developer, repulsiveforce can be given to the excess developer generated by the regulatingmember and the excess developer hardly binds to each other. As a result,circulation of the developer is performed smoothly.

Further, according to an example embodiment, one magnetic pole can beprovided so that its polar central axis is located in the opposite sideof the regulating member with respect to the minimum position of thesecond gap. Accordingly, the transportation amount of the developertransported to the second gap itself is reduced and excess developeramount generated by the regulating member can be reduced. As a result,the excess developer can be maintained in a condition that developer ishardly bound to each other.

According to example embodiment, one magnetic pole is provided so thatits polar central axis corresponds to the minimum position of the secondgap. Accordingly, the transportation amount of the developer transportedto the second gap is kept from too much increase and excess developergenerated by the regulating member can be maintained in a condition thatdeveloper is hardly bound to each other.

According to an example embodiment, one magnetic pole is provided sothat its polar central axis is in the regulating side with respect tothe minimum position of the second gap. Accordingly, the transportationamount of the developer transported to the second gap is kept from toomuch increase and excess developer generated by the regulating membercan be maintained in a condition that developer is hardly bound to eachother.

According to an example embodiment, since the width of the magnetic poleis set G1<G2≦0.8×Dm, a state of high magnetic flux density near therubbing member is obtained so that preliminary electrification isperformed efficiently.

According to an example embodiment, since the developer accumulationpreventing member for preventing accumulation of developer transportedto the gap is provided, the magnetic flux density in the circulationpath of the excess developer generated by the regulating member can bereduced and the developer is circulated smoothly. In addition,deterioration of the developer by unnecessary stress of developeraccumulation can be prevented.

According to example embodiment, since the rubbing member and the refluxplate reinforce each other, bending of the rubbing member toward gapexpanding direction caused by a drag of rubbing the developer can bereduced and the amount of the second gap is maintained. Further, bendingof the reflux plate can be reduced, so that circulating ability of thedeveloper is maintained.

According to an example embodiment, since the convex portion of thedeveloper accumulation preventing member contacts with the reflex plateto prevent bending of the rubbing member caused by developer, bending ofthe rubbing member toward gap expanding direction by a drag of rubbingthe developer can be reduced and the amount of the second gap ismaintained. Further, bending of the reflux plate can be reduced, so thatcirculating ability of the developer is maintained.

According an example embodiment, since difference in deformation of theregulating member and the rubbing member caused by temperature changescan be reduced, deformation of the first gap and the second gap can bereduced. Accordingly, stable first gap and second gap can be obtained.

According to an example embodiment, since the accurate positionalrelation among the developing roller, the regulating member, the rubbingmember can be maintained, the amount of the first gap and second gap canbe maintained. Further, the developing roller, the regulating member,the rubbing member can be integrated into a unit so that proper workingof the assembly is improved.

According to an example embodiment even when the average oftransportation amount M2av is lower than or equal to the second gapG2·(M1/G1), a part of the developer contacts with the regulating memberto be rubbed so that circulation amount by preliminary electrificationcan be increased. According to an example embodiment, since the thirdgap G3 is equal to or larger than 4.2 mm, the circulation resistance ofthe developer at the third gap is reduced and the reduction of thedeveloper circulation amount caused by circulation path resistance canbe controlled.

According to an example embodiment, since the third gap is larger thanthe second gap, the circulation resistance of the developer at the thirdgap is reduced and the reduction of the developer circulation amountcaused by circulation path resistance can be controlled.

According to an example embodiment, when the particle size of thedeveloper is reduced to improve the image quality, it is particularlyeffective since proper toner concentration of the developer becomeshigh, magnetic attraction force is reduced, and more developer dropsjust below the regulating member.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing a structure of a developingdevice according to an example embodiment;

FIG. 2 is a magnified view of a main part of the developing deviceaccording to an example embodiment;

FIG. 3 is an exploded perspective view showing a supporting structurewhich supports a developing roller and other elements;

FIG. 4 is a view showing scalar amount of magnetic flux densitycorresponding to distance in a radial direction on a polar central axisand displacement angle from a minimum position (a) of the polar centralaxis;

FIG. 5 is a view showing component in the radial direction of magneticflux density corresponding to distance in the radial direction on apolar central axis and displacement angle from the minimum position (a)of the polar central axis;

FIG. 6 is a view showing component in a rotational direction of magneticflux density corresponding to distance in the radial direction on apolar central axis and displacement angle from the minimum position (a)of the polar central axis;

FIG. 7( a) is a view showing magnetic flux density of component in theradial direction corresponding to displacement angle from a minimumposition (a) of the polar central axis;

FIG. 7( b) is a view showing normalized magnetic flux density ofcomponent in the radial direction corresponding to displacement anglefrom the minimum position (a) of the polar central axis;

FIG. 8( a) is a view showing magnetic flux density of component in therotational direction corresponding to displacement angle from theminimum position (a) of the polar central axis;

FIG. 8( b) is a view showing normalized magnetic flux density ofcomponent in the rotational direction corresponding to displacementangle from the minimum position (a) of the polar central axis;

FIGS. 9 are a view and a table showing the relationship between distancein the radial direction and the normalized magnetic flux density on thepolar central axis;

FIG. 10 are a view and a table showing the relationship between thelocation of the polar central axis with respect to the minimum position(a) of a second gap and developer circulation amount;

FIG. 11 is a view showing a relationship between a third gap anddeveloper circulation amount;

FIG. 12 is a vertical section front view showing a structure of a mainpart of an image forming apparatus including the developing deviceaccording to an example embodiment; and

FIG. 13 is a cross-sectional view showing a structure of a conventionaldeveloping device.

BEST MODE

An example embodiment of the present invention will be described withreference to the drawings.

FIG. 1 is a cross-sectional view showing a structure of a developingdevice according to an example embodiment.

The developing device includes a hopper 1 for containing two-componentdeveloper, a developing roller 3 for transporting the developer to anelectrostatic latent image held by a photosensitive drum 2 anddeveloping the electrostatic latent image, a regulating member 4 forregulating the amount of the developer transported to the electrostaticlatent image with the developing roller 3, a rubbing member 5 for facingthe developing roller 3 at a second gap G2 which is larger than a firstgap G1 defined by the developing roller 3 and the regulating member 4,regulating layer thickness of the developer transported to the first gapG1 to rub the developer, a reflux plate 6 for refluxing excess developerregulated by the regulating member 4 away from the regulating member 4,and an agitating roller 7 for agitating the developer in the hopper 1.Here, the regulating member 4, rubbing member 5, and the refluxing plate6 are formed in a length corresponding to the length of the developingroller 3.

The hopper 1 is formed in almost prismatic shape including an opening lain a position facing a circumference of the photosensitive drum 2 and adeveloper supply unit 1 b in a position away from the opening 1 a. Thedeveloping roller 3 and the regulating member 4 are provided in aposition facing the opening la of the hopper 1 and the agitating roller7 is rotatably provided in a position facing the developer supply unit 1b. Further, a transporting roller 8 is rotatably provided between thedeveloper supply unit 1 b and the agitating roller 7, for transportingdeveloper (toner) supplied into the hopper 1 through the developersupply unit 1 b to the agitating roller 7. A magnetic permeabilitysensor 9 for detecting toner density in the hopper 1 is provided on thedownside of the agitating roller 7. Accordingly, in case that toneramount agitated by the agitating roller 7 becomes lower than the properamount, toner may be supplied through the developer supply unit 1 b inaccordance with the detected value.

The developing roller 3 includes a magnet roller 31 as a multi-polemagnetization in which magnetic poles NI, N2, N3 and magnetic poles S1,82, 83 made of bar magnets having rectangle cross sectional shapes arearranged separately from each other and radially in the circumferentialdirection, and a nonmagnetic sleeve 32 which is rotatably fitted to themagnet roller 31. The magnet roller 31 has its both ends nonrotatablysupported by the walls of the hopper 1. The magnetic pole Ni is arrangedin a position facing the circumference of the photosensitive drum 2 andthe magnetic pole N2 is arranged in a fashion facing the second gap G2.The magnetic poles N1 and N2 have polar central axes P1 and P2 incenters in their widths in the circumferential direction of thedeveloping roller 3 throughout their entire length.

The magnetic pole N1 facing the photosensitive drum 2 is arranged sothat the polar central axis P1 is displaced 3 degrees toward upperstream of developer transportation with respect to a line which runsthrough a central axis O1 of the photosensitive drum 2 and a centralaxis O2 of the developing roller 3. The bias amount of the polar centralaxis P1 of the magnetic pole N1 is measured by a measure. This measurehas a magnetic guide which rotates about the central axis O1 of thephotosensitive drum 2 and the bias amount is detected in accordance withthe rotational angle of the guide.

FIG. 2 is a magnified view of a main pan. The magnetic pole N2 facingthe second gap G2 is positioned so that the polar central axis P2 ispositioned toward the regulating member 4 with respect to the minimumposition (a) of the second gap G2, in other words, positioned closer tothe regulating member 4 (downstream of developer transportation) withrespect to the position displaced 1.5 degrees toward the upper stream ofdeveloper transportation. Further, the width size Dm of the magneticpole N2 in the circumferential direction of the developing roller 3 isset to 4 mm.

The regulating member 4 carries out main electrification of thedeveloper as regulating the amount of developer transportation with thedeveloping roller 3. The regulating member 4 is made of a nonmagneticmetal plate having a rectangle cross sectional shape and one surface ofthe regulating member 4 in its width direction faces the circumferenceof the developing roller 3 at the first gap G1. The coefficient oflinear expansion of the regulating member 4 is almost the same as thatof the rubbing member 5. In other words, the regulating member 4 and therubbing member 5 are made of similar material and even when theregulating member 4 and the rubbing member 5 are displaced withcurvature result from temperature changes at both ends in longitudinaldirection, the changes in the first gap G1 and the second gap G2 can bereduced. Further, the regulating member 4 is located inside of theopening 1 a of the hopper 1 and between the magnetic pole S1 and themagnetic pole N2. The regulating member 4 is fixed to the opening 1 athrough a cover body 10 which is fixed to the regulating member 4. Here,the regulating member 4 and the rubbing member 5 are made of metalplates such as aluminum or stainless steel.

The rubbing member 5 carries out preliminary electrification of thedeveloper as regulating the transportation amount (layer thickness) ofdeveloper transported to the first gap G1 with the developing roller 3and rubbing the developer. The rubbing member 5 is integrally formedwith the nonmagnetic reflux plate 6. The reflux plate 6 is arranged soas to be a slope from the upper portion of the developing roller 3toward the upper portion of the agitating roller 7 in a manner that thedeveloping roller 3 is higher than the agitating roller 7. The end inthe developing roller 3 side of the reflux plate 6 is folded toward thedeveloping roller 3 so as to form a folded section 6 a. The foldedsection 6 a is defined as the rubbing member 5.

The rubbing member 5 is formed flat and the developing roller 3 isformed in a round shape so that the minimum position (a) is defined inthe second gap G2.

The relationship between the second gap G2, the first gap G1 (mm), andthe width Dm of the magnetic pole N2 (mm) is set as below.G1<G2≦0.8×Dm

It is preferable that the first gap G1 is typically set as 0.5 mm andthe second gap G2 is typically set as 2.3 mm or 3.2 mm. The width Dm ofthe magnetic pole N2 is set as 4 mm as mentioned above, however, thiswidth is not limited if it is a proper width.

The relationship among the first gap G1, the second gap G2, developeramount M1 (g/s/cm) per unit length transported from the first gap G1,and developer amount M2 (g/s/cm) per unit length transported from thesecond gap G2 is set as below.M2>(M1/G1)G2

Here, the M1 and M2 are amounts of developer transported through thegaps for 10 seconds and the measured length between the regulatingmember and the rubbing member in the longitudinal direction is 5 cm.(M1/G1) is a weight (g) of the developer transported through the gap perunit length of the gap.

A first developer accumulation preventing member 12 for preventingdeveloper transported to the first gap G1, that is, excess developerregulated by the regulating member 4, from accumulating in upper streamof the regulating member 4 is provided adjacent to the regulating member4 in the hopper 1. Further, a second developer accumulation preventingmember 13 for preventing developer transported to the second gap G2 fromaccumulating in upper stream of the rubbing member 5 is fixed adjacentto the rubbing member 5.

The first developer accumulation preventing member 12 and the seconddeveloper accumulation preventing member 13 are made of nonmagneticmaterials such as metal or synthetic resin having a length correspondingto the developing roller 3. The first developer accumulation preventingmember 12 is arranged between an end of the regulating member 4 in thewidth direction and an upper wall of the hopper 1 and the developeraccumulation preventing member 12 is separated from the reflux plate 6in the vertical direction so as to lead the excess developer to thereflux plate 6 smoothly. In addition, a plurality of convex portions 12a are provided separately from each other in a longitudinal direction atthe bottom part of the developer accumulation preventing member 12 so asto contact with the upper surface of the reflux plate 6. The convexportion 12 a prevents the reflux plate 6 from bending upwardly. Here, inFIG. 1, the developer accumulation preventing member 12 is laid from theregulating member 4 to an end of the reflux plate 6, however, the lengthof the developer accumulation preventing member 12 from the regulatingmember 4 is not limited. Further, the developer accumulation preventingmember 12 may be integrally formed with the regulating member 4.

The second developer accumulation preventing member 13 is providedbetween the rubbing member 5 and the reflux plate 6 along the lowersurface of the reflux plate 6, so that the developer may be smoothlytransferred to the second gap G2. The second developer accumulationpreventing member 13 may be integrated into the reflux plate 6.

FIG. 3 is an exploded perspective view showing a supporting structurefor supporting the developing roller and other elements. The developmentroller 3, the regulating member 4, and the reflux plate 6 (and rubbingmember 5) provided in the hopper I as described above are supported by acouple of common supporting members 11, 11. The supporting members 11,11 are composed of flat plates and include, in their central portions,large-diameter through holes 11 a, 11 a to which axis portions 3 a inboth sides of the developing roller 3 are fitted. in addition, thesupporting members 11, 11 includes, in their one ends, small-diameterthough holes 11 b, 11 b which face a plurality of screw holes 12 b, 12 bprovided in both ends of the first developer accumulation preventingmember 12 fixed to the regulating member 4 and smalldiameter throughholes 11 c, 11 c which face a plurality of screw holes 13 a, 13 aprovided in both ends of the second developer accumulation preventingmember 13 fixed to the rubbing member 5. Then, the axis portions 3 ainboth sides of the developing roller 3 are fitted to the thorough holes11 a, 11 a and external screws such as machine screws are tightenedthrough the screw holes 11 b, 11 c to the screw holes 12 b, 13 a in astate that the regulating member 4 and the rubbing member 5 are arrangedbetween the supporting members 11, 11. As a result, a unit is formed andthe unit is installed in the hopper 1.

In the hopper 1 of the developing device having above describedstructure, two-component developer is contained. The developer iscomposed of carrier such as magnetic powder and toner. The developerhaving small particle size smaller than typical particle size is used.The average particle size of typical carrier is 85 μm, and the averageparticle size of the carrier used in example embodiment is smaller thanor equal to 65 μm. Further, the average particle size of typical toneris 8.5 μm, and the average particle size of the toner used in exampleembodiment is smaller than or equal to 7.5 μm.

When an electrostatic latent image held by the photosensitive drum 2 isdeveloped, the photosensitive drum 2 rotates in clockwise direction asshown in FIG. 1 and the sleeve 32 of the developing roller 3 and theagitating roller 7 rotate in counterclockwise direction as shown inFIG. 1. The developer in the hopper 1 is transported to the second gap62 by the developing roller 3 as being agitated by the agitating roller7 and regulated the transportation amount (layer thickness) in thesecond gap G2. Then, the developer passes through the second gap G2 asbeing rubbed by the rubbing member 5. The developer transported to thesecond gap 62 is regulated in its accumulation by the second developeraccumulation preventing member 13 and is transported smoothly to thesecond gap G2. After passing though the second gap G2, the developer istransported to the first gap G1. The second gap G2 is formed larger thanthe first gap G1 so that excess developer is certainly generated by theregulating member 4 and a certain developer circulation amount isprovided by the rubbing member 5. The excess developer is regulated inits accumulation by the first developer accumulation preventing member12 and refluxed toward the refiux plate 6, and refluxed toward the upperpart of the agitating roller 7 by the reflux plate 6. As a whole, thedeveloper circulates around the reflux plate 6.

The second gap G2 is defined by the rubbing member 5 for rubbing thedeveloper with the developing roller 3 so that it is able to carry outpreliminary electrification on the developer when the developer passesthrough the second gap G2. The regulating member 4 regulates thetransportation amount of the preliminarily electrified developer so thatthe regulating member 4 can perform main electrification on thepreliminarily electrified developer. Further, the magnetic pole N2 isarranged so that the polar central axis P2 of the magnetic pole N2 islocated in side of the lower transportation stream than the positionwhich is displaced 1.5 degrees toward upper transportation stream withrespect to the minimum position (a) of the second gap G2. As a result,as shown in FIGS. 4, 5, 6 and 7 magnetic force toward the polar centralaxis P2 of the magnetic pole N2 is generated in the second gap G2.

FIG. 4 is a view showing scalar amount of magnetic flux densitycorresponding to distance in a radial direction on a polar central axisand displacement angle of the polar central axis from a minimum position(a). FIG. 5 is a view showing component in the radial direction ofmagnetic flux density corresponding to distance in the radial directionon a polar central axis and displacement angle of the polar central axisfrom a minimum position (a). FIG. 6 is a view showing component in arotational direction of magnetic flux density corresponding to distancein the radial direction on a polar central axis and displacement angleof the polar central axis from a minimum position (a). It is noted that,in FIGS. 4 to 6, the distance in the radial direction is sum of theradius size of the developing roller 3 and size of the second gap G2.

FIG. 7( a) is a view showing magnetic flux density of component in theradial direction corresponding to displacement angle of the polarcentral axis from the minimum position (a). FIG. 7( b) is a view showingnormalized magnetic flux density of component in the radial directioncorresponding to displacement angle of the polar central axis from theminimum position (a). The positive numbers on the horizontal axisindicate displacement angle in the upper transportation stream from theminimum position (a) and the negative numbers on the horizontal axisindicate displacement angle in the lower transportation stream from theminimum position (a). In FIGS. 7( a) and 7(b), component of the magneticflux density can be changed by changing the size of the second gap G2 onthe polar central axis P2 and the displacement angle of the polarcentral axis P2 from the minimum position (a). Further, as shown in FIG.7( b), when the width of the magnetic pole N2 Dm=4 mm, it can he made inone {paragraph }by normalizing the component of the magnetic fluxdensity. In FIGS. 7( a) and 7(b), the radius size of the developmentroller 3 is set 25 mm. The sizes corresponding to the sum of the radiussize of 25 mm and the sizes of the second gap G2, 2.5 mm, 3.5 mm, and7.0 mm indicate distance in the radial direction of the developingroller 3, 27.5 mm, 28.5 mm, and 32.0 mm.

FIG. 8( a) is a view showing magnetic flux density of component in therotational direction corresponding to displacement angle of the polarcentral axis from the minimum position (a). FIG. 8( b) is a view showingnormalized magnetic flux density of component in the rotationaldirection corresponding to displacement angle of the polar central axisfrom the minimum position (a). The positive numbers on the horizontalaxis indicate displacement angle in the upper transportation stream fromthe minimum position (a) and the negative numbers on the horizontal axisindicate displacement angle in the lower transportation stream from theminimum position (a). The magnetic flux density of the component inrotational direction is normalized as shown in FIG. 8( a) so that it canbe made in one {paragraph}as shown in FIG. 8( b).

FIGS. 9( a) and 9(b) are views showing the relationship between distancein the radial direction and normalized magnetic flux density on thepoiar central axis. The width Dm of the magnetic pole N2 is 4 mm and thesecond gap G2 is arranged so that G1<G2≦0.8×Dm. As G2≦0.8 ×Dm is set 2.3mm ×3.2 mm, the magnetic flux density B near the reflux plate 6 becomeshigh (B≧41%). In other words, magnetic attraction force which isproportional to the square of the magnetic flux density is added to thecarrier of the developer (B²≧17%) and the layer thickness is regulatedat the end of lower transportation stream at the rubbing member 5 in acondition under relative large frictional force, and hence preliminaryelectrification can be performed efficiently. Further, preferably, it isset at G2≦(⅔)·Dm, that is, 2.3 mm 2.7 mm, to provide a condition thatthe magnetic flux density B near the reflux plate 6 is relatively high(B≧45%), that is, a condition that magnetic attraction force which isproportional to the square of the magnetic flux density is added to thecarrier (B²≧20%).

The first gap G1, the second gap G2, developer amount per unit lengthM1, and developer amount per unit length M2 are set as M2>(M1/G1)G2.Accordingly, the developer transported to the second gap G2 is notregulated by the rubbing member 5 and the layer thicknesses are uneven.As a result, even when the average value of transportation amount M2avis lower than or equal to the second gap G2 (M1/G1), a part of thedeveloper contacts with the regulating member 4 to be rubbed andcirculation amount by the preliminary electrification can be increased.In the condition with the value lower than or equal to the regulationamount, the rubbing condition may be uneven because it heavily dependson the transportation amount and the developer layer forming condition.However, the unevenness can be prevented by the above settings. FIGS.10( a) and 10(b) are views showing the relationship between the locationof the polar central axis regarding the minimum position (a) of a secondgap and developer circulation amount.

As the developer circulation amount is influenced by the developeramount in the hopper 1 and the distance of the second gap, in FIGS. 10(a) and 10(b) ,the developer circulation amount is measured in acondition that the developer amount in the hopper 1 is 1400 g or 1250 gand the distance of the second gap G2 is 4 mm, 3.2 mm, or 2.3 mm. Untilthe polar central axis P2 of the magnetic pole N2 moves to the positiondisplaced 1.5 degrees toward the upper transportation stream from theminimum position (a) where the distance of the second gap G2 is minimum,magnetic force is attracted toward the polar central axis P2 in theupper transportation stream with respect to the minimum position (a) andthe transportation amount of the developer by the magnetic force in thesecond gap G2 becomes smaller than the case when displacing toward thelower transportation stream from the minimum position (a). As a result,a stable developer circulation amount can be obtained as shown in FIGS.10( a) and 10(b).

The developer circulation amount gradually increases as shown in FIG. 10by arranging the magnetic pole N2 so that the polar central axis P2 isdisplaced toward the lower transportation stream from the minimumposition (a). However, when the second gap G2 is 2.3 mm, the rubbingmember 5 starts to regulate the developer transportation amount incondition that the polar central axis P2 is displaced 1.5 degrees towardthe lower transportation stream from the minimum position (a). As aresult, the developer circulation amount can be stable even when thepolar central axis P2 is displaced more than 1.5 degrees toward thelower transportation stream from the minimum position (a). Further, whenthe second gap G2 is 4 mm, the rubbing member 5 hardly regulates thetransportation amount of the developer so that the developmentcirculation amount cannot be increased as compared to the case in whichthe second gap G2 is 3.2 mm or 2.3 mm.

Similar to that, as shown in FIGS.(10 a) and (10 b), while the rubbingmember 5 hardly regulates the developer circulation amount when thesecond gap G2 is 4 mm, the rubbing member 5 regulates the developercirculation amount when the second gap G2 is 3.2 mm or 2.3 mm, thecirculation path resistance in the third gap G3 drastically increases bythe rubbing of the first developer accumulation preventing member 12 andthe developer, the developer circulation amount drastically reduces whenthe third gap G3 becomes smaller than 4 mm.

FIG. 12 is a vertical section front view showing a structure of a mainpart of an image forming apparatus including the developing deviceaccording to example embodiment.

The developing device having the above described structure is installedin, for example, an image forming apparatus having anelectrophotographic process unit. The image forming apparatus is, asshown in FIG. 12, a digital copying machine including a scanner 20 forscanning an image of a manuscript, a rotative photosensitive drum 2(image forming unit) having an electrostatic latent image on itscircumference, an electrification means 21 for performingelectrification on the photosensitive drum 2, an exposing means having alaser beam scanner for making the photosensitive drum 2 hold theelectrostatic latent image corresponding to the image of manuscript, adeveloping device A for developing the electrostatic latent image, atransferring means 22 for transferring the developed toner image on thephotosensitive drum 2 on a sheet, a cleaning means for removingdeveloper remained in the photosensitive drum 2, an electricity removingmeans for removing electrification of the photosensitive drum 2, a sheetfeeding unit 23 for feeding a sheet toward the photosensitive drum 2,and a sheet post-treatment unit 24 for post-treating the sheet on whichan image is formed.

According to the above described embodiment, a rubbing member isconfigured with a reflux plate in one plate shape, however, the rubbingmember may be provided separately from the reflux plate.

1. A developing device comprising: a developing roller having a magnetroller configured to transport developer to be used in developing anelectrostatic latent image; a regulating member configured to regulatean amount of the developer transported to the electrostatic latent imagewith the development roller; a reflux plate configured to reflux excessdeveloper regulated by the regulating member away from the regulatingmember; and a rubbing member arranged to face the developing roller at asecond gap which is larger than a first gap defined by the developingroller and the regulating member, wherein the rubbing member isconfigured to rub developer transported to the first gap in order tocreate friction prior to preliminary electrification of the developer;and wherein the magnet roller has a plurality of magnetic poles arrangedin circumferential direction thereof and one of the magnetic poles isprovided so that a polar central axis thereof faces the second gap andthe polar central axis is positioned between the regulating member and aposition upstream of a direction of developer transportation that is 1.5degrees from a minimum position of the second gap; and wherein thesecond gap G2 (mm) G1<G2≦0.8×Dm, where G1 is the first gap (mm) and Dmis a width of magnetic pole (mm).
 2. The developing device according toclaim 1, wherein the magnetic pole is arranged so that the polar centralaxis is positioned between the regulating member and the minimumposition of the second gap.
 3. The developing device according to claim1, further comprising a developer accumulation preventing memberpositioned near the regulating member and/or the rubbing memberconfigured to prevent accumulation of developer transported.
 4. Thedeveloping device according to claim 3, wherein the reflux plate and thedeveloper accumulation preventing member provided near the regulatingmember define a third gap.
 5. The developing device according to claim4, wherein the third gap is larger than the second gap.
 6. Thedeveloping device according to claim 1, wherein the rubbing member isintegrally formed with the reflux plate.
 7. The developing deviceaccording to claim 6, wherein the developer accumulation preventingmember provided near the regulating member has a convex portion whichcontacts with the reflux plate and is configured to prevent thedisplacement of the rubbing member by developer.
 8. The developingdevice according to claim 1, wherein the regulating member and therubbing member have similar coefficients of linear expansion.
 9. Thedeveloping device according to claim 1, wherein the developing roller,the regulating member, and the rubbing member are supported by a commonsupporting member.
 10. The developing device according to claim 1,wherein amount of developer transported to the second gap M2 (g/s/cm) isdefined asM2>(M1/G1)G2 where G1 is the first gap (mm), G2 is the second gap (mm),and M1 is the amount of developer transported from the first gap(g/s/cm).
 11. The developing device according to claim 1, furthercomprising a hopper containing developer including magnetic powderhaving an average particle size is smaller than or equal to 65 μm andtoner having an average particle size smaller than or equal to 7.5 μm.12. A developing device, comprising: a developing roller having a magnetroller configured to transport developer to be used in developing anelectrostatic latent image; a regulating member configured to regulatean amount of the developer transported to the electrostatic latent imagewith the development roller; a reflux plate configured to reflux excessdeveloper regulated by the regulating member away from the regulatingmember; and a rubbing member arranged to face the developing roller at asecond gap which is larger than a first gap defined by the developingroller and the regulating member, wherein the rubbing member isconfigured to rub developer transported to the first gap, in order tocreate friction prior to preliminary electrification of the developer;and wherein the magnet roller has a plurality of magnetic poles arrangedin circumferential direction thereof and one of the magnetic poles isprovided so that the magnetic pole faces the second gaps and a polarcentral axis thereof corresponds to a minimum position of the secondgap; and wherein the second gap G2 (mm) G1<G2≦0.8×Dm, where G1 is thefirst gap (mm) and Dm is a width of magnetic pole (mm).
 13. Thedeveloping device according to claim 12, further comprising a developeraccumulation preventing member configured to prevent accumulation ofdeveloper transported to the gap near the regulating member and/or therubbing member.
 14. The developing device according to claim 13, whereinthe reflux plate and the developer accumulation preventing memberprovided near the regulating member define a third gap.
 15. Thedeveloping device according to claim 14, wherein the third gap is largerthan the second gap.
 16. The developing device according to claim 12,wherein the rubbing member is integrally formed with the reflux plate.17. The developing device according to claim 16, wherein the developeraccumulation preventing member provided near the regulating member has aconvex portion which contacts with the reflux plate and is configured toprevent the displacement of the rubbing member by developer.
 18. Thedeveloping device according to claim 12, wherein the regulating memberand the rubbing member have similar coefficients of linear expansion.19. The developing device according to claim 12, wherein the developingroller, the regulating member, and the rubbing member are supported by acommon supporting member.
 20. The developing device according to claim12, wherein amount of developer transported to the second gap M2(g/s/cm) is defined asM2>(M1/G1)G2 where G1 is the first gap (mm), G2 is the second gap (mm),and M1 is the amount of developer transported from the first gap(g/s/em).
 21. The developing device according to claim 12, furthercomprising a hopper containing developer including magnetic powderhaving average particle size smaller than or equal to 65 μm and tonerhaving an average particle size smaller than or equal to 7.5 m.
 22. Adeveloping device, comprising: a developing roller having a magnetroller configured to transport developer to be used in developing anelectrostatic latent image; a regulating member configured to regulatean amount of the developer transported to the electrostatic latent imagewith the development roller; a reflux plate configured to reflux excessdeveloper regulated by the regulating member away from the regulatingmember; and a rubbing member arranged to face the developing roller at asecond gap which is larger than a first gap defined by the developingroller and the regulating member, wherein the rubbing member isconfigured to rub developer transported to the first gap in order tocreate friction prior to preliminary electrification of the developer;and wherein the magnet roller has a plurality of magnetic poles arrangedin circumferential direction thereof and one of the magnetic poles isprovided so that the magnetic pole faces the second gap and a polarcentral axis thereof is located between a position corresponding to theminimum position of the second gap and a position displaced 1.5 degreeupstream from the minimum position of the second gap in a direction ofdeveloper developer transportation; and wherein the second gap G2 (mm)is defined as G1<G2≦0.8×Dm, where G1 is the first gap (mm) and Dm is awidth of magnetic pole (mm).
 23. The developing device according toclaim 22, further comprising a developer accumulation preventing memberconfigured to prevent accumulation of developer transported to the gapnear the regulating member and/or the rubbing member.
 24. The developingdevice according to claim 23, wherein the reflux plate and the developeraccumulation preventing member provided near the regulating memberdefine a third gap.
 25. The developing device according to claim 24,wherein the third gap is larger than the second gap.
 26. The developingdevice according to claim 22, wherein the rubbing member is integrallyformed with the reflux plate.
 27. The developing device according toclaim 26, wherein the developer accumulation preventing member providednear the regulating member has a convex portion which contacts with thereflux plate and is configured to prevent the displacement of therubbing member by developer.
 28. The developing device according toclaim 22, wherein the regulating member and the rubbing member havesimilar coefficients of linear expansion.
 29. The developing deviceaccording to claim 22, wherein the developing roller, the regulatingmember, and the rubbing member are supported by a common supportingmember.
 30. The developing device according to claim 22, wherein amountof developer transported to the second gap M2 (g/s/cm) is defined asM2>(M1/G1)G2 where G1 is the first gap (mm), G2 is the second gap (mm),and M1 the amount of developer transported from the first gap (g/s/cm).31. The developing device according to claim 22, further comprising ahopper containing developer including magnetic powder having an averageparticle size smaller than or equal to 65 μm and toner having an averageparticle size smaller than or equal to 7.5 μm.
 32. An image formingapparatus, comprising: a developing device of claim 1 developing anelectrostatic latent image; and an image forming unit configured to forma developed image on a sheet.
 33. An image forming apparatus,comprising: a developing device of claim 12 developing an electrostaticlatent image; and an image forming unit configured to form a developedimage on a sheet.
 34. An image forming apparatus, comprising: adeveloping device of claim 22 developing an electrostatic latent image;and an image forming unit configured to form a developed image on asheet.
 35. The image forming apparatus of claim 4, wherein the third gapis equal to or larger than 4.2 mm.
 36. The image forming apparatus ofclaim 14, wherein the third gap is equal to or larger than 4.2 mm. 37.The image forming apparatus of claim 24, wherein the third gap is equalto or larger than 4.2 mm.